This invention relates to solid state trip circuits for circuit breakers and more particularly relates to a novel saturable transformer structure and solid state trip circuit in which tripping at high overload is derived from the fault current whereas a direct connection from the line voltage to the trip circuit controls relatively low overload tripping.
Circuit breakers containing solid state trip circuits which imitate the behavior of conventional thermal magnetic trip units for circuit breakers are well known and are disclosed, for example, in U.S. Pat. No. 4,064,469 and in the references described therein. The line current in such circuits is normally monitored by a current transformer. Current transformers used to date have been relatively large to avoid distortion of the line current and to insure linearity which might be lost due to the saturation of the transformer. Other circuits have been devised to permit a saturating transformer to be used with solid state trip circuits, but these have distorted the waveform of the primary bus and have been relatively large. Moreover, it is frequently required that two transformers be used, one for deriving control power to be used to power the entire solid state trip unit, while the other is used to derive a measure of the primary current which is used to operate the sensing and timing circuits of the solid state trip unit.